i18n: fix coding tag unsupported by xgettext

Running `make update-pot` currently fails with the following error:

xgettext: mercurial/metadata.py:1: Unknown encoding "utf8". Proceeding with ASCII instead.
xgettext: Non-ASCII string at mercurial/metadata.py:311.
          Please specify the source encoding through --from-code or through a comment
          as specified in http://www.python.org/peps/pep-0263.html.

Differential Revision: https://phab.mercurial-scm.org/D9260

--HG--
branch : stable
13 jobs for 5.6 in 35 minutes (queued for 4 minutes and 53 seconds)
latest
Status Job ID Name Coverage
  Test
passed #118375
checks-py2

00:03:47

passed #118376
checks-py3

00:05:45

passed #118377
rust-cargo-test-py2

00:02:30

passed #118378
rust-cargo-test-py3

00:02:34

passed #118379
rust-cargo-test-py3-dirstate-tree

00:02:11

passed #118380
test-py2

00:26:29

passed #118387
test-py2-chg

00:22:22

passed #118382
test-py2-pure

00:19:05

passed #118384
test-py2-rust

00:21:31

passed #118381
test-py3

00:13:23

passed #118383
test-py3-pure

00:19:42

passed #118385
test-py3-rust

00:22:24

passed #118386
test-py3-rust-dirstate-tree

00:22:17